1. Filed of the Invention
The present invention generally relates to memory circuits and systems, and more particularly, the present invention relates to devices and methods for controlling active termination resistors which are used improve signaling characteristics in memory circuits and systems.
2. Description of the Related Art
Generally, as the bus frequency of a memory system (e.g., a memory system employing DRAM devices) increases, the signal integrity within the memory system degrades. Thus, a variety of bus topologies capable of reducing signal distortion have been developed. For example, it is known that the use of resistive terminations at either the receiver and/or transmitter sides within the memory system is an effective means for absorbing reflections and thereby improving signal performance. Resistive termination configurations of this type generally fall into one of two categories, i.e., passive termination or active termination.
FIG. 1 shows an example of a passive resistive termination in a memory system. In particular, a so-called stub series terminated logic (SSTL) standard is illustrated in which the bus of a memory system 100 is connected to termination voltages Vterm through termination resistors Rterm, and DRAM-mounted memory modules are inserted into slots having predetermined stub resistors Rstub. In this case, the stub resistors Rstub are not mounted on the DRAM chips, and accordingly, the example here is one of an xe2x80x9coff-chipxe2x80x9d passive resistive termination.
When used in a double data rate (DDR) memory system, the passive resistive termination of the SSTL standard is capable of ensuring a data rate of about 300 Mbps. However, any increase in data rate beyond 300 Mbps tends to degrade signal integrity by increasing the load of the bus having the resistive stubs. In fact, a data rate of 400 Mbps or greater is generally not achievable with the SSTL bus configuration.
FIG. 2 shows an example of a memory system having an active resistive termination, and in particular, an active-termination stub bus configuration. Here, each chipset for controlling the operation of the memory modules, and DRAMs mounted on the respective modules, includes an active termination resistor Rterm. The active termination resistor Rterm is mounted xe2x80x9con-chipxe2x80x9d and may be implemented by complementary metal oxide semiconductor (CMOS) devices. In this memory system, active bus termination is achieved through input/output (I/O) ports mounted on the modules.
Each combination of one or more resistive elements Rterm and one or more ON/OFF switching devices in each DRAM is generally referred to herein as an xe2x80x9cactive terminatorxe2x80x9d. Active terminators can take on any number of different configurations, and FIG. 3 illustrates an example of an active terminator having a center-tapped termination which is described in U.S. Pat. No. 4,748,426. In this example, the effective Rterm of the circuit can be varied between different values (e.g., 150 ohms and 75 ohms) depending on the enable/disable state of signals ON/OFFxe2x80x941 and ON/OFFxe2x80x942.
When a DRAM mounted in a memory module is not accessed (e.g., not read or written), the active termination resistor Rterm thereof is enabled by connecting the same to the bus to improve signal integrity. In contrast, when a DRAM is accessed (e.g., read or written), the active termination resistor Rterm thereof is disabled and disconnected from the bus to reduce load.
However, a considerable amount of time is required to enable the active termination resistors installed in the DRAM circuits in response to the active termination control signals, and when a module-interleaved write/read operation is performed, this time lapse can result in data bobbles, thereby degrading memory system performance. DRAMs which include a delay locked loop (DLL) or phase locked loop (PLL) can overcome this problem by controlling the enabling/disabling of the active termination resistor thereof in synchronization with an external clock. However, in the case where the DLL or PLL is deactivated during a power down or standby mode of a corresponding memory module, enabling/disabling of the active termination resistor cannot be controlled.
According to one aspect of the present invention, a buffer circuit is mounted in a memory circuit and includes a signal terminal, a synchronous input buffer having an input coupled to said signal terminal, an asynchronous input buffer having an input coupled to said input terminal, and a switching circuit which selectively outputs an output of said synchronous input buffer or an output of said asynchronous input buffer according to an operational mode of the memory circuit.
According to another aspect of the present invention, an active termination circuit is mounted in a memory circuit and includes a termination resistor which provides a termination resistance for the memory circuit, and a control circuit which receives an externally supplied active termination control signal, and which selectively switches on and off the termination resistor in response to the active termination control signal. The control circuit includes a synchronous input buffer and an asynchronous input buffer which each receive the active termination control signal, and a switching circuit which selectively outputs an output of said synchronous input buffer or an output of said asynchronous input buffer according to an operational mode of the memory circuit. The output of the switching circuit controls an on/off state of said termination resistor.
According to still another aspect of the present invention, an active termination circuit is mounted in a memory circuit and includes a termination resistor which provides a termination resistance for the memory circuit, a mode register which stores data indicative of an operational mode of the memory circuit, and a control circuit which receives an externally supplied active termination control signal and an output of the mode register. The control circuit includes a synchronous input buffer and an asynchronous input buffer which each receive the active termination control signal, and a switching circuit which selectively outputs an output of said synchronous input buffer or an output of said asynchronous input buffer according to the output of the mode register. The output of the switching circuit controls an on/off state of the termination resistor.
According to yet another aspect of the present invention, a memory system includes a bus line, a plurality of memory circuits coupled to the bus line, and a chip set, coupled to the bus line, which supplies a plurality of active termination control signals to the memory circuits. Each of the plurality of memory circuits includes a termination resistor and a control circuit. The control circuit receives the active termination control signal supplied to the memory circuit thereof, and selectively switches on and off the termination resistor in response to the active termination control signal. Further, the control circuit includes a synchronous input buffer and an asynchronous input buffer which each receive the active termination control signal, and a switching circuit which selects one of an output of the synchronous input buffer or an output of the asynchronous input buffer according to an operational mode of the memory circuit containing the buffer circuit. The output of the switching circuit controls an on and off state of the termination resistor.
According to another aspect of the present invention, a memory system includes a bus line, a plurality of memory circuits coupled to the bus line, and a chip set, coupled to said bus line, which supplies a plurality of active termination control signals to the memory circuits. Each of the plurality of memory circuits includes a termination resistor, a control circuit, and a mode register which stores data indicative of an operational mode of the memory circuit. The control circuit includes a synchronous input buffer and an asynchronous input buffer which each receive the active termination control signal, and a switching circuit which selects one of an output of the synchronous input buffer or an output of said asynchronous input buffer according to the data of the mode register. The output of the switching circuit controls an on/off state of the termination resistor.
According to another aspect of the present invention, a method for controlling an operation of a memory circuit includes applying an input signal to a synchronous input buffer and to an asynchronous input buffer of the memory circuit, and selectively outputting an output of the synchronous input buffer or an output of the asynchronous input buffer according to an operational mode of the memory circuit.
According to a further aspect of the present invention, a method of controlling an on/off state of a termination resistor of a memory circuit includes supplying an active termination control signal to both a synchronous input buffer and an asynchronous input buffer of the memory circuit, selecting an output of the synchronous input buffer when the memory circuit is in an active operational mode, and selecting an output of the asynchronous input buffer when the memory circuit is in a standby or power-down operational mode, and setting an on/off state of the termination resistor according to the selected one of the output of the synchronous input buffer or the output of the asynchronous input buffer.
According to a further aspect of the present invention, a method is provided for controlling a plurality of termination resistors of a respective plurality of memory circuits in a memory system, where the memory system has a plurality of memory modules connected to a data bus. Each of the memory modules for mounting at least one of the plurality of memory circuits thereto. The method includes supplying an active termination control signal to both a synchronous input buffer and an asynchronous input buffer of each of the memory circuits of each of the memory modules, selecting, in each memory circuit, an output of the synchronous input buffer when the memory circuit is in an active operational mode, and selecting an output of the asynchronous input buffer when the memory circuit is in a standby or power-down operational mode, and setting, in each memory circuit, an on/off state of the termination resistor according to the selected one of the output of the synchronous input buffer or the output of the asynchronous input buffer.
According to still another aspect of the present invention, a method is provided for controlling a plurality of termination resistors of a respective plurality of memory circuits in a memory system, where the memory system having at least a first memory module and a second memory module connected to a data bus, and each of the memory modules is for mounting at least one of the plurality of memory circuits thereto. The method includes transmitting, in response to a read/write instruction of the first memory module, an active termination control signal to the memory circuits of each of the second memory module, supplying the active termination control signal to both a synchronous input buffer and an asynchronous input buffer of each of the memory circuits of the second memory module, selecting, in each of the memory circuits of the second memory module, an output of the synchronous input buffer when the second memory module is in an active operational mode, and selecting an output of the asynchronous input buffer when the second memory module is in a standby or power-down operational mode, and setting, in each memory circuit of the second memory module, an on/off state of the termination resistor according to the selected one of the output of the synchronous input buffer or the output of the asynchronous input buffer.